Method for heat-treating bainite steel rail

ABSTRACT

A method for heat-treating a bainite steel rail is disclosed. The method includes: cooling a rolled steel rail naturally to lower a surface temperature of a rail head of the steel rail to 460° C. −490° C.; cooling the steel rail forcely at a cooling rate of 2.0° C./s-4.0° C./s to lower the surface temperature of the rail head to 250° C.-290° C.; placing the steel rail in an ambient temperature until the surface temperature of the rail head is more than 300° C.; performing a tempering on the steel rail in a heating furnace at 300° C.-350° C. for 2 h-6 h; and air cooling the steel rail to the ambient temperature. Steel rails heat-treated by present method may have a stable retained austenite structure and a good mechanical performance.

TECHNICAL FIELD

The present disclosure relates to a method for heat-treating a bainite steel rail, more particularly, to a method for heat-treating a high-performance bainite steel rail containing stable retained austenite.

BACKGROUND ART

A steel rail is a critical component for guiding trains and transferring a wheel load to a track bed. The quality of the steel rail directly influences operation efficiency and security of a railway. In recent years, with continuing increase of axel load and carrying gross weight of the railway, a steel rail of higher performance is required.

Researches have proved that a carbide-free bainite/martensite dual-phase steel composed of bainite type ferrite, retained austenite and a trace of martensite has great potential in the field of steel rail, and a lot of work has been done in researching, manufacturing and applying a bainite steel rail. At present, processes of manufacturing a bainite steel rail mainly include:

(1) rolling a billet into a steel rail, and then directly air cooling the same to an ambient temperature, so that a complex phase steel rail of upper baisite (trace)+carbide free baisite+retained austenite (a small amount)+martensite (trace) being obtained, and the strength and toughness of the steel rail being improved by stabilizing the retained austenite of the steel rail and transforming the martensite into tempered martensite through a subsequent tempering process compared to a air cooling.

(2) increasing contents of metals such as Mo, Ni, V, Ti, etc. in a steel rail, and directly air cooling a rolled steel rail to an ambient temperature, so that a complex phase steel of carbide free baisite+retained austenite+martensite having good strength and toughness being obtained.

Since the method does not include an austenite stabilizing step, the retained austenite in the steel rail tends to be transformed into a brittle marstensite by external forces such as impact of train wheels, etc.

(3) rolling a billet into a steel rail, beginning to apply a cooling medium onto the steel rail at a phase region of austenite so as to acceleratedly cool the steel rail to 300-500° C. at a cooling rate of 1-10° C./s, and air cooling the steel rail to an ambient temperature, so that a bainite steel rail having good strength and toughness being obtained. The method was disclosed by patent application of CN1095421A. Researches indicate that the steel rail produced by this method tends to be bent and deformed due to a non-uniform distribution of thermal capacity of cross section of the steel rail in the accelerated cooling process, which reduces the straightness of the steel rail, and a relatively large residual stress will be formed in a subsequent straightening process, thereby influencing safety of the steel rail.

SUMMARY

The purpose of the present disclosure is to overcome the above defects of prior arts and provide a method for heat-treating a bainite steel rail.

The method for heat-treating the bainite steel rail according to the present disclosure comprises: cooling a rolled steel rail naturally to lower a surface temperature of a rail head of the steel rail to 460° C.-490° C.; cooling the steel rail forcely at a cooling rate of 2.0° C./s-4.0° C./s to lower the surface temperature of the rail head to 250° C.-290° C.; placing the steel rail in an ambient temperature until the surface temperature of the rail head is more than 300° C.; performing a tempering on the steel rail in a heating furnace at 300° C.-350° C. for 2 h-6 h; and air cooling the steel rail to the ambient temperature.

According to an exemplary embodiment of the present disclosure, cooling the steel rail forcely may be performed by applying a cooling medium onto the rail head.

According to an exemplary embodiment of the present disclosure, the cooling medium may include an air-water gas or a compressed air.

Steel rails heat-treated by present method may have a stable retained austenite structure and a good mechanical performance.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects and advantages of the present disclosure will become apparent and easily understood by reference to the following description of exemplary embodiments in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic view of section hardness test positions of a rail head according to China Railway Industry Standard.

FIG. 2 is a microstructure photograph of a bainite steel rail obtained by a heat-treating method according to an exemplary embodiment of the present disclosure.

FIG. 3 is a microstructure photograph of a bainite steel rail obtained by a conventional heat-treating method.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

In a rolling process of a bainite steel rail, a billet containing components of a bainite steel rail is usually fed into a heating furnace and heated at a soaking temperature of 1200° C.-1300° C. for no less than 2 h, and a fast-to-slow heating mode is usually adopted for the soaking. Then, the billet is rolled into a steel rail of a required section in a rolling mill after being kept at a certain temperature for a predetermined time. Furthermore, the final rolling temperature is usually about 900° C.-1000° C.

Hereinafter, a method of heat-treating a steel rail (for example, a steel rail obtained by the above rolling) will be described in details by referring to exemplary embodiments of present disclosure. However, the present disclosure is not limited to heat-treating the above rolled steel rail.

First, a rolled steel rail is cooled down naturally to lower a surface temperature of a rail head to about 460° C.-490° C., and then, the steel rail is cooled down forcely at a cooling rate of about 2.0° C./s-4.0° C./s to lower the surface temperature of the rail head to about 250° C.-290° C.

According to an exemplary embodiment of the present disclosure, the rolled steel rail may be erected behind a roller table and air cooled until the surface temperature of the rail head reduces to about 460° C.-490° C., and then the steel rail may be cooled down forcely by applying a cooling medium onto the rail head, for example, by applying a cooling medium onto top surface and two side surfaces of the rail head. Here, the cooling rate of the steel rail may be controlled to be about 2.0° C./s-4.0° C./s. However, those skilled in the art will realize that the forced cooling of the steel rail is not limited thereto. In addition, according to an exemplary embodiment of the present disclosure, the cooling medium may include a mixed gas of water and air or a compressed air.

In the case that a steel rail is air cooled after being rolled, a phase transformation temperature of a bainite steel rail is about 350-400° C. In prior arts, an accelerated cooling begins at a phase region of austenite, as there is a wide range from the accelerated cooling temperature to the phase transformation temperature of bainite steel rail, a relatively long cooling time is required, and too much cooling medium will be consumed. In addition, in the case that the accelerated cooling begins at the phase region of austenite, when the surface of the rail head is acceleratedly cooled by the cooling medium in the process of accelerated cooling, heat from core of the rail head and rail web diffuses to the surface of the rail head via heat transfer, so that the rail head is difficult to complete phase transformation at a larger degree of under cooling and the toughness of the rail head decreases from the surface to the core, thus a complete hardening can not be achieved. Therefore, there is no benefit to significantly improving the performance of the rail steel by performing the accelerated cooling at a temperature between the phase region of austenite and 490° C. However, according to the exemplary embodiment of the present disclosure, when the surface of the rail head is cooled naturally down to about 460° C.-490° C., the temperatures of the rail web and rail bottom are less than 500° C. When the accelerated cooling is performed at this time, the surface temperature of the rail head has been decreased prominently, and the heat from the core of the rail head cannot effectively compensate for the surface of the rail head. Meanwhile, as the temperature of 460° C.-490° C. is relatively close to the phase transformation temperature of bainite, the entire section, especially the core of the rail head is allowed to complete the phase transformation at a larger degree of under cooling. Therefore, the steel rail heat-treated by the method according to the exemplary embodiment of the present disclosure may have better performances, compared to that heat-treated by a conventional method.

In addition, if the cooling rate is less than 2° C./s in the process of forced cooling, the surface temperature of the rail head is difficult to decrease quickly, thus the temperature of the core of the rail head can not be reduced effectively. Meanwhile, the heat from the core of the rail head may be transferred to the surface of the rail head, which is not favorable to improve the overall performance of the steel rail. If the cooling rate is more than 4° C./s, too much martensite is generated due to the high cooling rate of the surface of the rail head. Although the martensite may be transformed into tempered marstensite by a subsequent tempering process, the tranformation is not complete, and the residual marstensite structure is not beneficial to the safe use of the steel rail.

In addition, if the final temperature of the surface of the rail head is more than 290° C. after the forced cooling process, although a fine bainite structure is formed in the surface of the rail head, a coarse bainite structure will be formed at the core of the rail head due to high temperature, which may finally degrade performance of the steel rail at ambient temperature, and is not beneficial to uniformity of performance in the entire section of the rail head. If the final temperature of the surface of the rail head is less than 250° C., a large amount of martensite is generated in bainite structure, which is difficult to be removed by a subsequent tempering process, so that toughness and plasticity of the steel rail decrease prominently, and even the steel rail cannot be used.

Next, the steel rail is placed in an ambient temperature until the surface temperature of the rail head is more than 300° C. Then, a tempering is performed on the steel rail in a heating furnace at 300° C.-350° C. for 2 h-6 h.

According to an exemplary embodiment of the present disclosure, the surface temperature of the rail head may rise by 50-60° C. due to the heat from the core of the rail head and the rail web, when the steel rail is placed in air after the forced cooling process. Therefore, the surface temperature of the rail head may naturally rise to be more than 300° C. after the above forced cooling to 250° C.-290° C. When the above forced cooling is completed, the heat from the rail web and the core of the rail head may still be transferred to the surface of the rail head, that is, the entire section of the steel rail is in a soaking status. When the average temperature of the steel rail is about 300-350° C. after a period of soaking, and the tempering is performed on the steel rail, so that the time for tempering may be reduced prominently and a more uniform performance may be obtained. In addition, a fine bainite structure has been formed through the above forced cooling, retained austenite is included between lamellas of lamellate bainite type ferrite, and the retained austenite is not stable, which needs to be further stabilized by tempering, so as to obtain stable steel rail having good toughness and strength. Although the temperature rise of the steel rail after the forced cooling process can play the role of tempering to some extent, the function is limited. The reason is that the heat from the rail web and rail bottom may only keep a short period of temperature rise, and when the steel rail reaches a soaking state, the temperature of the entire section decreases simultaneously to an ambient temperature in a very short time. Therefore, tempering relied on temperature rise of the steel rail itself has limited influence on the structure, and a part of the retained austenite in the steel rail is still in a metastable state. Therefore, the tempering process by heating at about 300° C.-350° C. is adopted in the present disclosure.

According to an exemplary embodiment of the present invention, the temperature of the heating furnace is about 300° C.-350° C. When the tempering temperature is less than 300° C., the toughness and plasticity of the steel rail, especially the impact toughness at −40° C. are obviously decreased, thus the high-toughness feature of the bainite steel rail at low temperature does not take effect. When the tempering temperature is more than 350° C., although the toughness and plasticity are improved, the strength and hardness tend to decrease, which is not beneficial to obtain steel rails having good overall performance. In addition, when the tempering time is less than 2 h, a part of the retained austenite in the steel rail is in a metastable state, and the purpose of stabilizing the retained austenite can not be achieved. When the tempering time is more than 6 h, the transformation of the retained austenite in the steel rail is completed, i.e., the purpose of tempering is achieved, thus there is no obvious benefit to prolonging the tempering time.

At last, the tempered steel rail is air cooled to an ambient temperature, so as to obtain a steel rail having a stable retained austenite structure and a good mechanical performance.

Hereinafter, the method for heat-treating the high-performance bainite steel rail having a stable retained austenite structure is described in details in conjunction with particular exemplary embodiments of present disclosure.

Table 1 shows chemical components of bainite steel rails according to exemplary embodiments of the present disclosure and comparative examples, however, the heat treatment method of the present disclosure is not limited to be used for the steel rails having the chemical components in table 1.

TABLE 1 Chemical Components (wt %) C Si Mn P S Cr Mo Embodiment 1 and 0.23 1.58 1.97 0.010 0.006 0.80 0.30 Comparative Example 1 Embodiment 2 and 0.25 1.20 2.10 0.011 0.005 1.22 — Comparative Example 2 Embodiment 3 and 0.26 1.75 1.65 0.011 0.007 0.50 0.36 Comparative Example 3 Embodiment 4 and 0.21 0.80 1.95 0.014 0.009 1.05 0.32 Comparative Example 4 Embodiment 5 and 0.24 1.10 2.05 0.012 0.004 0.95 0.37 Comparative Example 5 Embodiment 6 and 0.26 1.30 1.87 0.013 0.006 1.53 0.25 Comparative Example 6

Billets having the above components were rolled into steel rails of 60 kg/m, and heat-treating methods in table 2 were respectively applied to the steel rails, wherein a subsequent tempering was applied in the embodiments, while no tempering was applied in the comparative examples.

TABLE 2 Initial Final Temperature Temperature Temperature of Steel of of Rails when Temperature Accelerated Accelerated Accelerated being of Heating Cooling Cooling Cooling feeding into Furnace Tempering (° C.) Rate (° C./s) (° C.) furnace (° C.) (° C.) Time (h) Embodiment 1 485 3.5 280 330 300 4.1 Embodiment 2 472 2.0 289 312 320 4.8 Embodiment 3 469 2.4 252 305 330 5.9 Embodiment 4 461 2.1 267 308 350 5.1 Embodiment 5 488 3.9 256 302 310 4.2 Embodiment 6 490 3.1 288 321 320 4.6 Comparative 760 1.8 350 — — — Example 1 Comparative 780 2.4 381 — — — Example 2 Comparative 820 2.2 364 — — — Example 3 Comparative 880 3.1 425 — — — Example 4 Comparative 690 2.9 346 — — — Example 5 Comparative 570 1.9 315 — — — Example 6

The steel rails were aired cooled to ambient temperature after the above processes, and mechanical properties thereof were shown in tables 3 and 4.

TABLE 3 Impact Property Tensile Property (Aku/J) Proportion Rp0.2 Ambient of Retained (MPa) Rm (MPa) A/% Z/% Temperature −40° C. Austenite Embodiment 1 1130 1380 18.5 62 95 68 6.3% Embodiment 2 1180 1410 16.5 58 85 59 7.2% Embodiment 3 1150 1430 17.0 62 87 61 7.8% Embodiment 4 1210 1490 17.5 56 98 64 7.1% Embodiment 5 1200 1390 17.5 56 102 72 6.6% Embodiment 6 1160 1420 19.0 50 88 58 6.9% Comparative 1025 1340 16.0 52 85 52 11.8% Example 1 Comparative 1040 1320 14.0 50 64 46 10.2% Example 2 Comparative 1080 1370 13.5 44 62 42 12.1% Example 3 Comparative 1105 1400 15.5 40 62 38 10.8% Example 4 Comparative 1060 1310 15.0 46 72 48 11.6% Example 5 Comparative 980 1300 16.0 42 60 40 10.0% Example 6

TABLE 4 Surface Section Hardness (HRC) Hardness A₁ B₁ C₁ A₄ B₅ C₅ (HBW) Embodiment 1 44.0 44.5 45.0 44.0 43.5 44.0 435 Embodiment 2 43.0 44.0 44.5 43.5 44.0 43.5 430 Embodiment 3 44.5 44.5 45.0 44.5 44.0 44.0 438 Embodiment 4 45.0 45.5 45.0 44.5 44.5 44.5 440 Embodiment 5 43.5 44.0 44.0 44.0 43.5 44.0 428 Embodiment 6 43.0 44.0 44.5 43.5 43.5 44.0 425 Comparative 41.5 42.0 42.0 40.0 40.5 41.0 415 Example 1 Comparative 41.0 42.0 41.5 41.5 42.0 41.5 418 Example 2 Comparative 42.0 41.5 41.5 41.0 41.0 40.5 420 Example 3 Comparative 42.5 42.0 42.5 41.5 42.0 42.0 425 Example 4 Comparative 40.5 40.0 40.5 40.0 40.0 40.5 406 Example 5 Comparative 39.5 40.0 40.0 39.5 40.0 39.5 398 Example 6

FIG. 1 is a schematic view of section hardness test positions of a rail head according to China Railway Industry Standard. The section hardness test points A₁, B₁, C₁, A₄, B₅, and C₅ of the rail head in table 4 is shown in FIG. 1, wherein A₁, B₁, C₁ respectively represent three positions of the surface of the rail head, and A₄, B₅, and C₅ respectively represent three positions in the core of the rail head.

FIG. 2 is a microstructure photograph of a bainite steel rail obtained by a heat-treating method according to an exemplary embodiment of the present disclosure. FIG. 3 is a microstructure photograph of a bainite steel rail obtained by a conventional heat-treating method.

Based on analyses of tables 1-4 and FIGS. 2 and 3, it can be seen that in the condition of same chemical components, and smelting and rolling processes, the method of heat-treating the rolled steel rail may have obvious influence on the final performance of the steel rail. Particularly, with the heat-treating method according to the embodiments of the present disclosure, the microstructure of the steel rail is a mixed structure composed of lath-shaped bainite ferrite, discrete retained austenite film alternatively distributed between the laths in a lamella shape, and a small amount of twin marstensite. In same eutectic cell, the bainite ferrite laths have uniform orientation, distance between laths is small, such fine structure can obviously improve strength and hardness of the steel rail and slightly improve toughness and plasticity of the steel rail, thus the function of the grain refinement strengthening to improve overall performance of the steel rail is sufficiently achieved. In addition, the decrease of the content of the retained austenite may obviously decrease the tendency of the retained austenite to be transformed into brittle marstensite by external force at ambient temperature, i.e., the retained austenite have a better mechanical stability, which is more beneficial to the reliability and safety in train operation. In comparison, when the conventional heat-treating method is adopted, although the structure is still composed of bainite ferrite, retained austenite, and a small amount of marstensite, since the final temperature of the accelerated cooling is relatively high, the entire section of the rail head is difficult to complete phase transformation at a larger degree of under cooling, causing that the structure in the steel rail is coarse, and a small amount of eutectoid ferrite is precipitated. Since the function of accelerated cooling to improve performance of the bainite steel rail cannot be sufficiently achieved, the steel rail has low hardness and strength as well as unsatisfying toughness and plasticity. Furthermore, in the conventional heat-treating method, the retained austenite has a high proportion, coarse size, and morphology of continuous and closed distribution, and tends to be transformed into marstensite under the impact of the wheels of trains, causing brittle failure of the steel rail, which may result in train operation accident. However, the steel rail heat-treated by the method of the embodiment of the present disclosure can not only meet higher performance requirements for railway steel rail, but also ensure safe operation of trains.

In conclusion, the method for heat-treating high-performance bainite steel rail of the present disclosure is applied to rolled steel rails having residual heat, in the condition of the same chemical components and smelting and rolling processes, the steel rails heat-treated by the method of the embodiments of the present disclosure have better strength and toughness compared to that heat-treated by conventional methods. In addition, the steel rails heat-treated by methods of embodiments of the present disclosure are suitable for heavy haul railways having relatively high requirements for contact fatigue damage and wearability.

Although the method for heat-treating a bainite steel rail has been described by referring to particular embodiments of the present disclosure, those skilled in the art should realize that within the spirit and scope of the present disclosure, various amendments and changes may be made. 

What is claimed:
 1. A method for heat-treating a bainite steel rail, comprising: cooling a rolled steel rail naturally to lower a surface temperature of a rail head of the steel rail to 460° C.-490° C.; cooling the steel rail forcely at a cooling rate of 2.0° C./s-4.0° C./s to lower the surface temperature of the rail head to 250° C.-290° C.; placing the steel rail in an ambient temperature until the surface temperature of the rail head is more than 300° C.; performing a tempering on the steel rail in a heating furnace at 300° C.-350° C. for 2 h-6 h; and air cooling the steel rail to the ambient temperature.
 2. The method for heat-treating the bainite steel rail of claim 1, wherein, cooling the steel rail forcely is performed by applying a cooling medium onto the rail head.
 3. The method for heat-treating the bainite steel rail of claim 2, wherein, the cooling medium includes an air-water gas or a compressed air. 